Protective circuit



Dec. 21, 1965 CLARK ETAL 3,225,254

PROTECTIVE CIRCUIT Filed June 2, 1961 2 Sheets-Sheet 1 TIP/665B 500905BY W0 Dec. 21, 1965 5. CLARK ETAL 3,225,254

PROTECTIVE CIRCUIT Filed June 2, 1961 2 Sheets-Sheet 2 79/6659 SOUQCE850965 4. CZAZQKZ JOE NJ AVG/Z54 INVENTORS a 9% BY Z Cyw/ United StatesPatent 3,225,254 PROTECTIVE CIRCUIT George L. Clark and John J. Hickey,both of Hawthorne, Caiifi, assignors, by mesne assignments, to TRW Inc.,a corporation of Ohio Filed June 2, 1961, Ser. No. 114,472 13 Claims.(Cl. 315-163) This invention relates to circuits employing controldevices, such as thyratron tubes, and more particularly to the provisionof means for protecting a trigger source from high voltage pulsesgenerated in a thyratron tube upon the firing thereof.

When a thyratron fires, the potential of the grid may momentarily jumpto a value nearly equal to the anode potential. This pulse of voltage,commonly known as the grid spike, is transmitted through the gridcoupling capacitor to the circuit which supplies the trigger to thethyratron grid. In the case of large thyratrons operating with anodepotentials of -30 kilovolts and carrying hundreds of amperes of current,the grid spike can cause considerable damage to a trigger circuit whichis not adequately protected.

The usual type of protection which is provided for the trigger circuitis a low pass filter inserted between it and the thyratron grid.Although the filter does cut down the amount of grid spike reaching thetrigger source, it also reduces the size and rate of rise of the triggerpulse which is sent to the thyratron grid, thereby making it necessaryto generate a larger trigger pulse than would be required in the absenceof the filter. Since the grid spike is about 100 times as large as thetrigger pulse, it is extremely difficult to filter out the grid spikeand still trigger the thyratron reliably.

Accordingly, it is an object of this invention to protect a signalsource from spurious high voltages generated in a control device that isfed from the signal source.

It is a more specific object of this invention to provide a protectivecircuit which operates to prevent the grid spike voltage of a thyratrontube from being transmitted to the trigger source supplying thethyratron with a trigger voltage pulse.

A further object is the prvoision of means for block ing thetransmission of grid spike voltages from a thyratron to the sourcesupplying a trigger pulse to the thyratron, without interfering with thetransmission of the trigger pulse.

The foregoing and other objects are realized, according to oneembodiment, in a protective circuit interposed between a trigger sourcesupplying a trigger voltage pulse and a control device, such as a mainthyratron tube, receiving said trigger pulse. The protective circuitincludes a switching device or protective thyratron in shunt with thetrigger source and normally biased nonconducting in the absence of atrigger pulse from the trigger source.

When a trigger pulse is supplied from the trigger source the pulse isreceived by the protective thyratron tube. However, since the protectivethyratron tube is provided with means for delaying the rise of its gridvoltage to the firing potential, it remains nonconducting for a shorttime sutiicient to permit transmission of the trigger pulse to the mainthyratron tube. The protective thyratron tube is arranged to fire beforethe main thyratron tube fires. Thus, when the main thyratron tube fires,the grid spike voltage generated therein is shunted through theprotective thyratron and is prevented from reaching the trigger source.A series impedance between the two thyratrons serves to limit thecurrent drawn by the second thyratron as a result of its being subjectedto the grid spike voltage.

3,225,254 Patented Dec. 21, 1965 In the drawings, wherein like numeralsare used to represent like parts:

FIG. 1 is a schematic representation of one form of a protective circuitaccording to the invention; and

FIGS. 2, 3, and 4 are schematic representations of other forms of aprotective circuit according to the invention.

Referring to FIG. 1 a trigger source 10, shown in block form, isprovided with a pair of output terminals 12 and 14 across which isgenerated a trigger voltage pulse 16 to be fed to a control device ormain thyratron tube 18. The trigger source may, for example, comprise aconventional blocking oscillator circuit and a cathode follower. Themain thyratron tube 18 may, for example, be one of the kind, such astype 1907, handling 20 kilovolts and carrying several hundred amperes ofcurrent. Normally it is desired to apply the trigger pulse 16 to thecontrol grid 20 of the main thyratron tube 18 to render the latterconducting, and thereby initiate some further action under the controlof the main thyratron tube 18, such as generating a pulse to drive ahigh-power tube, or discharging a capacitor. In this case, the mainthyratron tube 18 is used in a pulse forming circuit. The anode 22 ofthe main thyratron tube is connected to a high voltage source 24 throughan anode resistor 26. The high voltage source 24 is of the order of 20kilovolts. A pulse forming network, such as a delay line 28, isconnected in the anode circuit to generate an output pulse of given timeduration across an output pulse transformer 30 also connected in theanode circuit. The screen grid 32 and cathode 34 are maintained atground reference potential.

When the main thyratron tube 18 is triggered by applying a positivepulse to the control grid 20, the region of the tube between the grid 20and cathode 34 ionizes first, and then the region between the grid 20and anode 22 ionizes. When the gas between the grid 20 and anode 22becomes conducting, the potential of the control grid 20 risesmomentarily to a value nearly equal to the potential of the anode 22 andthen falls to a potential a little higher than that of the cathode 34.It is necessary to provide some means for preventing this increase inpotential, or grid spike, as it is commonly known, from reaching thetrigger source 10, to avoid damage to the latter. The grid spike isshown in FIG. 1 as a sharply rising pulse 36.

In accordance with the invention, a protective circuit is interposedbetween the trigger source 10 and the main thyratron tube 18. Theprotective circuit is arranged to transmit with little attenuation thetrigger pulse 16 from the trigger source 10 to the control grid 20 ofthe main thyratron tube 18. However, once the trigger pulse 16 reachesthe control grid 20 of the main thyratron tube 18 and remains there longenough to subsequently trigger the main thyratron tube 18, theprotective circuit blocks the transmission of the grid spike 36 from themain thyratron tube 18 to the trigger source 10.

The protective circuit includes a first Series resistor 38 connected atone end to the output terminal 12 of the trigger source 10 and connectedat its other end to the anode 40 of a switching device or protectivethyratron tube 42. The protective thyratron tube ;42 may be a miniaturethyratron such as type 2'D21 that includes a cathode 44, a control grid46 and a screen grid 48, in addition to the anode 40. The screen grid 48is preferably tied to the cathode 44, which is connected to ground. Thecontrol grid 46 is biased beyond cutoff by connection through a gridbias resistor 50 to a negative bias voltage source 52, so that theprotective thyratron tube 42 is maintained normally in a nonconductingcondition. A series circuit comprising variable resistor 54 and a DC.blocking capacitor 56 is connected between the control grid 46 and theanode 40 of the second thyratron tube 42, and a capacitor 58 isconnected between the control grid 46 and cathode 44-.

A second series resistor 60 is connected between the anode 40 of theprotective thyratron tube 42 and the control grid 20 of the mainthyratron tube 18. A resistor 60, connected between ground and the anode40 of the protective thyratron tube 42 serves to return the grid 20 ofthe main thyratron 18 and the anode 40 of the protective thyratron 42 toground potential between trigger pulses.

In the operation of the circuit of FIG. 1, both thyratr-on tubes 18 and42 are normally nonconducting in the absence of the trigger pulse 16from the trigger source 10. When it is desired to render the mainthyratron tube 18 conducting the trigger pulse 16 is sent from thetrigger source through the first series resistor 38, and the secondseries resistor 60 to the control grid 20 of the main thyratron tube 18.The three resistors 54, 50, and 62 shunting the protective thyratrontube 42 are sufiiciently large in resistance so that they do not affectthe transmission of the trigger pulse 16. The series resistors 38 and 60are much lower in resistance, the first series resistor 38 serving toprotect the trigger source 10 from being short circuited when theprotective thyraton tube 42 is rendered conducting, and the secondseries resistor 60 serving to limit the current drawn by the protectivethyratron tube 42 when the grid spike voltage 36 is impressed upon theprotective thyratron tube 42.

The trigger pulse 16 from the trigger source 10 is impressed across theanode 40 of the protective thyratron tube 42 and the control grid 20 ofthe main thyratron tube 18 almost simultaneously. When it is impressedupon the anode 40 of the protective thyratron tube 42, the capacitor 58charges up through the variable resistor 54 and capacitor 56, therebyraising the control grid potential of the protective thyratron tube 42.Capacitor 56 is much larger than capacitor 58, so that a negligibleportion of the trigger pulse 16 appears across capacitor 56. After acertain amount of delay, which is determined by the time constant of thecapacitor 58 and variable resistor 54, the latter being variable toadjust the time constant, the potential on the control grid 46 of thesecond thyratron tube 42 reaches the firing potential, whereupon theprotective thyratron tube 42 is rendered conducting. The delay in thefiring of the protective thyratron tube 42 is arranged to be shorterthan the delay in the firing of the main thyratron tube 18. Normallylarge thyratrons such as the main thyratron tube 18 have a delay time inexcess of 70 nanoseconds, whereas miniature thyratrons such as theprotective thyratron tube 42, in the absence of special delay circuitry,have delay times less than 70 nanoseconds.

When the protective thyratron tube 42 fires, it shunts and cuts off thetrigger pulse 16 if the latter has not already been terminated. Theduration of the trigger pulse 16 need only be long enough to cause theeventual firing of the main thyratron tube 18. After a certain length oftime following the firing of the protective thyratron tube 42, the mainthyratron tube 18 fires, whereupon an output pulse is generated acrossthe transformer 30 and a grid spike appears at the control grid 20. Thegrid spike voltage 36 is transmitted through the second series resistorand is shunted by the low resistance of the protective thyratron tube 42that is now conducting. The grid spike voltage 36 is thereby preventedfrom reaching the trigger source 10.

The second series resistor 60 insures that the protective thyratron tube42 will not be overloaded by current during the occurrence of the gridspike 36. The coupling capacitor 56 provides D.C. isolation between thecontrol grid 46 of the protective thyratron 42 and the rest of thecircuit.

In one operative embodiment, circuit values were as follows:

Resistor 50 10,000 ohms.

Capacitor 58 micro-microfarads. Source 52 50 volts.

Resistor 54 10,000 ohm potentiometer. Tube 42 Type 2D21 thyratron. Tube18 Type 1907 thyratron. Capacitor 56 0.02 microfarad.

Resistor 62 10,000 ohms.

Resistor 60 ohms.

Resistor 38 50 ohms.

In some instances, the inherent delay in the firing of the protectivethyratron tube 42 may be sufficient to permit the trigger pulse 16 tolast long enough to energize the main thyratron tube 18. In suchinstances, the delay circuit of FIG. 1 may be dispensed with. As shownin the embodiment of FIG. 2 the capacitor 56, capacitor 58, the biassource 52, and grid bias resistor 50 are eliminated, and the variableresistor 54 is replaced by a fixed resistor 64. Otherwise, the circuitis the same as that of FIG. 1. Depending upon the amount of triggervoltage provided by the trigger source 10, which voltage may be between200 and 1000 volts, a delay of from 0.05 to l microsecond may berealized in the firing of the protective thyratron tube 42. A triggerpulse of this duration is sufiicient to trigger the main thyratron tube18. A longer delay occurs in the firing of the main thyratron tube 18 inorder to assure that the protective thyratron tube 42 fires before themain thyratron tube 18, so that the former will be conducting when thegrid spike voltage occurs and will thereby be in condition to shunt thegrid spike voltage.

It is generally desirable in thyratron control circuits to utilize arather long duration trigger pulse to trigger a thyratron, such as themain thyratron tube 18, in order to reduce the jitter time, or variationin the switching time of the tube. Furthermore, it may be desirable toutilize the same tube type for each of the main thyratron and protectivethyratron tubes. One form of pro tective circuit which fulfills theserequirements is shown in FIG. 3. In this circuit a protective thyratron70 is used in conjunction with a main thyratron tube 72 which may be ofthe same tube type as the protective thyratron tube 70. For example,each of the tubes 70 and 72 may be a type 4C3 5 A delay network 74,comprising a plurality of series inductors 76 and shunt capacitors 78,is connected between the anode 80 of the protective thyratron tube 70and the junction of the resistors 60 and 62. The control grid 82 of theprotective thyratron tube 70 is connected to the output of the delaynetwork 74 at the junction of the resistors 60 and 62.

In operation, the trigger pulse 16 is delayed from reaching the controlgrid 82 of the protective thyratron 80 and the control grid 84 of themain thyratron tube 72. Assuming the tubes 70 and 72 firesimultaneously, it can be seen that the grid spike 36, which must passthrough the delay network 74 before reaching the anode 80 of theprotective thyratron, will be delayed by the network 74. Thus, the delaynetwork 74 assures that the protective thyratron tube 70 will beconducting prior to reception of the grid spike 36. Accordingly, thegrid spike 36 is shunted through the protective thyratron 70 before itcan reach the trigger source 10.

It can be seen that the amount of delay produced by the delay network 74can be adjusted to render the protective circuit of FIG. 3 effectiveeven in instances where the protective thyratron tube 70 fires laterthan the main thyratron tube 72. Complete protection can be realized inthis instance if the delay of the delay network 74 is greater than thedifference in the firing times of the two thyratrons 70 and 72.Accordingly, even ditferent tube types can be used for the thyratrons 70and 72, provided the above requirement is met. For example, a tube typecan be used for the protective thyratron 70 that has an inherent delaygreater than that of the tube type used for the main thyratron 72.

Another circuit which is useful for extending the duration of thetrigger pulse and for affording complete protection despite a longerinherent delay in the firing of the protective thyratron relative to themain thyratron is shown in FIG. 4. In this embodiment, a trigger source90 provides a main trigger pulse 16 for energizing the main thyratrontube 72 and a separate auxiliary trigger pulse 92 for energizing theprotective thyratron tube 70. The auxiliary trigger pulse 92 is delayedwith respect to the main trigger pulse 16, the delay being built intothe trigger source 90. The auxiliary trigger pulse 92 is fed through acoupling capacitor 94 to the control grid 82 of the protective thyratrontube 70. A negative bias voltage applied to the control grid 82 througha bias resistor 96 from a bias source 98 maintains the protectivethyratron tube 70 normally nonconducting until the auxiliary triggerpulse 92 renders it conducting.

A delay network 74 is connected between the series resistor 60 and theshunt resitor 62 to delay the arrival of the grid spike voltage relativeto the firing of the protective thyratron 70.

The separate trigger pulses 16 and 92 should be spacedapart in time onlyas long as is necessary to provide a main trigger pulse 16 of suflicientduration to trigger the main thyratron tube 72. The duration of the maintrigger pulse 16 is equal to the sum of the delay between the twotrigger pulses 16 and 92 plus the inherent delay in the firing of theprotective thyratron tube 70. The relationship between the inherentdelay (A) in the firing of the protective thyratron 70, the inherentdelay (B) in the firing of the main thyratron 72, the delay (5) providedby the delay network 74 and the delay (T) between the two trigger pulses16 and 92 is determined by the following expression:

Adherence to the foregoing relationship will assure that the arrival ofthe grid spike voltage will be delayed until after the firing of theprotective thyratron tube 70.

It is now apparent that the invention provides an effective means fortransmitting operative signal pulses from a signal source to a controldevice, while shunting spurious signal from the control device through aswitching device, thereby blocking the spurious signals from the signalsource. The one-way transmission is elfected through the use of aswitching device that operates on a time delay basis, rather than on athreshold voltage basis, as in some prior art devices. A switchingdevice in the form of a thyratron makes the invention particularlyeffective in applications requiring very high current and voltagehandling capabilities.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In combination, an electrical control device having an input circuitand adapted to change in electrical properties after the application tothe input circuit thereof of an electrical control signal of at least apredetermined time duration, thereby giving rise in said input circuitto a spurious signal; a terminal for receiving said control signal;means for coupling said control signal from said terminal to the inputcircuit of said control device; and means for blocking the transmissionof said spurious sig nal from said input circuit of said control deviceto said terminal, said last mentioned means comprising a switchingdevice connected in shunt relation to said terminal, a current limitingimpedance connected in series between said switching device and saidinput circuit of said control device, time delay means for maintainingsaid switching device at a relatively high impedance, after applicationto said terminal of said control signal, at least for said predeterminedtime duration, means for transforming said switching device to arelatively low impedance, and means for delaying the arrival of saidspurious signal at said terminal until after the transformation of saidswitching device.

2. The invention according to claim 1, wherein said switching deviceincludes an input circuit, and said time delay means comprises a delaynetwork connected in series with said current limiting impedance, withthe end of said delay network remote from said switching deviceconnected to said input circuit.

5. In combination, a main thyratron tube having an input circuit andadapted to change in electrical properties after the application to theinput circuit thereof of an electrical control signal of at least apredetermined time duration, thereby giving rise in said input circuitto a spurious signal; a terminal for receiving said control signal,means for coupling said control signal from said terminal to the inputcircuit of said main thyratron tube; and means for blocking thetransmission of said spurious signal from said input circuit of saidmain thyratron tube to said terminal, said last mentioned meanscomprising a protective thyratron tube connected in shunt relation tosaid terminal, a current limiting impedance connected in series betweensaid protective thyratron tube and said input circuit of said mainthyratron tube, time delay means for maintaining said protectivethyratron tube at a relatively high impedance, after application to saidterminal of said control signal, at least for said predetermined timeduration, means for transforming said protective thyratron tube to arelatively low impedance, and means for delaying the arrival of saidspurious signal. at said terminal until after the transformation of saidprotective thyratron tube.

6. The invention according to claim 5, wherein said main thyratron tubehas an inherent switching time that is greater than that of saidprotective thyratron tube.

7. The invention according to claim 6, wherein said protective thyratrontube includes a control grid and an anode coupled separately to saidterminal, and said time delay means comprises a resistor-capacitorcharging circuit coupled between said terminal and the control grid ofsaid protective thyratron tube.

8. The invention according to claim 6, wherein said protective thyratrontube includes a control grid and an anode coupled separately to saidterminal, with a resistor connected between said control grid and saidanode.

9. In combination, a main thyratron tube having an input circuit andadapted to switch within a first time period after the application tothe input circuit thereof of an electrical control signal of at least apredetermined time duration, thereby giving rise in said input circuitto a spurious signal; a terminal for receiving said control signal,means for coupling said control signal from said terminal to the inputcircuit of said main thyratron tube; and means for blocking thetransmission of said spurious signal from said input circuit of saidmain thyratron tube to said terminal, said last mentioned meanscomprising a protective thyratron having an anode and a control grid andconnected in shunt relation to said terminal, said protective thyratronhaving an inherent switching time smaller than that of said mainthyratron tube, a current limiting impedance connected in series betweensaid protective thyratron and said input circuit of said main thyratrontube, time delay means including a series resistance capacitance networkconnected between the anode and control grid of said protectivethyratron for maintaining said protective thyratron at a relatively highimpedance, after application to said terminal of said control signal,for a time at least equal to said predetermined time duration, and meansfor transforming said protective thyratron to a relatively lowimpedance, the switching time of said main thyratron tube being greaterthan the sum of the switching time of said protective thyratron tube andthe time delay of said time delay means, thereby to delay the arrival ofsaid spurious signal at said terminal until after the transformation ofsaid protective thyratron.

10. In combination, a main thyratron having an input circuit and adaptedto switch to a conducting state after the application to the inputcircuit thereof of an electrical control signal of at least apredetermined time duration, thereby giving rise in said input circuitto a spurious signal; a terminal for receiving said control signal,means for coupling said control signal from said terminal to the inputcircuit of said main thyratron tube; and means for blocking thetransmission of said spurious signal from said input circuit of saidmain thyratron tube to said terminal, said last mentioned meanscomprising a protective thyratron connected in shunt relation to saidterminal, a current limiting impedance connected in series between saidprotective thyratron and said input circuit of said main thyratron,means for applying a separate trigger pulse independent from saidcontrol signal to said protective thyratron, with said trigger pulsebeing delayed with respect to said control signal to maintain saidprotective thyratron at a relatively high impedance, after applicationto said terminal of said control signal, at least for said predeterminedtime duration, means for transforming said protective thyratron to arelatively low impedance, and means for delaying the arrival of saidspurious signal at said terminal until after the transformation of saidprotective thyratron.

11. The invention according to claim 10, wherein the means for delayingthe arrival of said spurious signal comprises a delay network connectedin series with said current limiting impedance.

12. In combination, a main thyratron having an input circuit and adaptedto change in electrical properties after the application to the inputcircuit thereof of an electrical control signal of at least apredetermined time duration, thereby giving rise in said input circuitto a spurious signal; a terminal for receiving said control signal,means for coupling said control signal from said terminal to the inputcircuit of said main thyratron; and means for blocking the transmissionof said spurious signal from said input circuit of said main thyratronto said terminal, said last mentioned means comprising a protectivethyratron connected in shunt relation to said terminal and including acontrol grid, a current limiting impedance connected in series betweensaid protective thyratron and said input circuit of said main thyratron,a delay network connected in series with said current limiting impedancewith the end of said delay network remote from said protective thyratronconnected to said control grid, thereby to maintain said switchingdevice at a relatively high impedance, after application to saidterminal of said control signal, at least for said predetermined timeduration, means for transforming said switching device to a relativelylow impedance, and means for delaying the arrival of said spurioussignal at said terminal until after the transformation of said switchingdevice.

13. The invention according to claim 12, wherein the sum of the inherentdelay of said main thyratron and the delay introduced by said delaynetwork is greater than the inherent delay of said protective thyratrontube, whereby said delay network serves to delay the arrival of saidspurious signal at said terminal until after the transformation of saidprotective thyratron.

References Cited by the Examiner UNITED STATES PATENTS 2,129,088 9/1938George 328l0 2,342,673 2/1944 Klemperer 315-272 X 2,572,832 10/1951Bernard 328-10 2,642,552 6/1953 Sager 315272 X GEORGE N. WESTBY, PrimaryExaminer.

RALPH NILSON, Examiner.

CHARLES R. CAMPBELL, Assistant Examiner.

10. IN COMBINATION, A MAIN THYRATRON HAVING AN INPUT CIRCUIT AND ADAPTEDTO SWITCH TO A CONDUCTING STATE AFTER THE APPLICATION TO THE INPUTCIRCUIT THEREOF OF AN ELECTRICAL CONTROL SIGNAL OF AT LEAST APREDETERMINED TIME DURATION, THEREBY GIVING RISE IN SAID INPUT CIRCUITTO A SPURIOUS SIGNAL; A TERMINAL FOR RECEIVING SAID CONTROL SIGNAL,MEANS FOR COUPLING SAID CONTROL SIGNAL FROM SAID TERMINAL TO THE INPUTCIRCUIT OF SAID MAIN THYRATRON TUBE; AND MEANS FOR BLOCKING THETRANSMISSION OF SAID SPURIOUS SIGNAL FROM SAID INPUT CIRCUIT OF SAIDMAIN THYRATRON TUBE TO SAID TERMINAL, SAID LAST MENTIONED MEANSCOMPRISING A PROTECTIVE THYRATRON CONNECTED IN SHUNT RELATION TO SAIDTERMINAL, A CURRENT LIMITING IMPEDANCE CONNECTED IN SERIES BETWEEN SAIDPROTECTIVE THYRATRON AND SAID INPUT CIRCUIT OF SAID MAIN THYRATRON,MEANS FOR APPLYING A SEPARATE TRIGGER PULSE INDEPENDENT, FROM SAIDCONTROL SIGNAL TO SAID PROTECTIVE THYRATRON, WITH SAID TRIGGER PULSEBEING DELAYED WITH RESPECT TO SAID CONTROL SIGNAL TO MAINTAIN SAIDPROTECTIVE THYRATRON AT A RELATIVELY HIGH IMPEDANCE, AFTER APPLICATIONTO SAID TERMINAL OF SAID CONTROL SIGNAL, AT LEAST FOR SAID PREDETERMINEDTIME DURATION, MEANS FOR TRANSFORMING SAID PROTECTIVE THYRATRON TO ARELATIVELY LOW IMPEDANCE, AND MEANS FOR DELAYING THE ARRIVAL OF SAIDSPURIOUS SIGNAL AT SAID TERMINAL UNTIL AFTER THE TRANSFORMATION OF SAIDPROTECTIVE THYRATRON.